Phase change materials, such as Ge2Sb2Te5 (GST), are used as the active recording media in current optical storage and upcoming solid state memories because of their remarkable properties. They can be rapidly and reversibly transformed between the amorphous and crystalline phases, and they exhibit large contrast in the optical and electrical properties between the two phases, which allows us to define bits of information. Understanding the structure of the amorphous phase is important in the development of phase change memory technologies, because nucleation, the first stage of crystallization, is dependent on the amorphous structure.
In this dissertation, we first analyze the evolution of subcritical nuclei and the nucleation kinetics as a function of nitrogen alloying and thermal annealing in the amorphous phase change material Ge2Sb2Te5. The existence of subcritical nuclei is inferred through measurement of the nucleation time in pulsed laser annealing, and is detected more directly using fluctuation transmission electron microscopy (FTEM) measurements that are sensitive to topological order on the nanoscale. In samples that are pre-annealed before crystallization experiments, the nanoscale order consistently increases and the nucleation times consistently decrease, in agreement with the interpretation that the nanoscale order corresponds to a population of subcritical nuclei that ripens upon annealing. However, this correlation is less obvious in as-deposited samples across a range of nitrogen contents: the quantity of nanoscale order diminishes only slightly with increased nitrogen alloying, whereas the nucleation times increase by two orders of magnitude. In parallel, we have performed the first FTEM measurements of amorphous phase change materials GeTe and N-alloyed GeTe. In GeTe samples that are pre-annealed prior to crystallization, the nanoscale order increases, and is correlated with shorter nucleation times as observed in subsequent laser crystallization experiments. However, after nitrogen alloying, the nanoscale order remain the same but the nucleation time increases significantly. Due to the dependence of the nuclei population on the thermodynamics parameters, the current results suggest that the thermodynamic energies are not strongly altered. We therefore interpret that nitrogen must reduce the rate at which the stochastic events for nucleation and growth take place (the kinetics).
We also investigate the time dependence of low temperature annealing or of extended storage at room temperature on the subsequent nucleation behavior of as-deposited amorphous AgIn-incorporated Sb2Te (AIST). Interestingly, the effect of annealing is observed to saturate: there is no further reduction in nucleation time or increase in nanoscale order for annealing at 100°C beyond three hours. This result supports the general prediction of classical nucleation theory that the size distribution of subcritical nuclei increases from the as-deposited state (with less order) to a quasi-equilibrium.
Phase change alloys are by design metastable, poor glass-forming alloys, and hence the presence of order in the amorphous phase is expected. We therefore analyze the evolution of nanoscale order in amorphous GexSe1-x alloys, which display a poor to good glass-forming tendency as a function of composition x, using FTEM. We identify two distinct structural signatures that behave independently as a function of composition. The strong signature of order at scattering vectors k ~ 0.30 and 0.55 Å-1 in Ge-rich alloys (x > 0.40) diminishes rapidly in Se-rich compositions. However, a second signature of order at scattering vector k ~ 0.15 Å-1 appears only for compositions in the middle range x = 0.30 – 0.53. We interpret these results to indicate structural ordering among pure Ge tetrahedra and among GeSe4 tetrahedra in nominally amorphous GexSe1-x.